Improved bounds on the Hadwiger–Debrunner numbers

Abstract

Let HD d (p, q) denote the minimal size of a transversal that can always be guaranteed for a family of compact convex sets in Rd which satisfy the (p, q)-property (p ≥ q ≥ d + 1). In a celebrated proof of the Hadwiger–Debrunner conjecture, Alon and Kleitman proved that HD d (p, q) exists for all p ≥ q ≥ d + 1. Specifically, they prove that $$H{D_d}(p,d + 1)is\tilde O({p^{{d^2} + d}})$$ . We present several improved bounds: (i) For any $$q \geqslant d + 1,H{D_d}(p,d) = \tilde O({p^{d(\frac{{q - 1}}{{q - d}})}})$$ . (ii) For q ≥ log p, $$H{D_d}(p,q) = \tilde O(p + {(p/q)^d})$$ . (iii) For every ϵ > 0 there exists a p0 = p0(ϵ) such that for every p ≥ p0 and for every $$q \geqslant {p^{\frac{{d - 1}}{d} + \in }}$$ we have p − q + 1 ≤ HD d (p, q) ≤ p − q + 2. The latter is the first near tight estimate of HD d (p, q) for an extended range of values of (p, q) since the 1957 Hadwiger–Debrunner theorem. We also prove a (p, 2)-theorem for families in R2 with union complexity below a specific quadratic bound.